1. Field of the Invention
The invention relates to a method for the positioning of web-shaped recording substrates in printing devices.
2. Brief Description of the Background of the Invention Including Prior Art
If a movable, web-shaped recording substrate which exhibits scannable elements is to be precisely positioned in a printing device, for example in an ink jet printer, in a thermal transfer printer, in a dot pin printer, and in a laser printer, relative to a print station, then the drive device, responsible for the positioning, has to be laid out and constructed correspondingly in order to avoid positioning errors. Tolerances in the drive device are predominantly the cause for the positioning errors, where the reached target position of the movable, web-shaped recording substrate, exhibiting scannable elements, deviates from the desired position. If the language relates in the following to a web-shaped recording substrate, then this designates a recording substrate, where the recording substrate can differ both in the kind (for example paper, cardboard, foil) as well as in the kind of the scannable elements, for example edge perforation hole, dash-code-like or bar-code-like stripe.
A typical application situation for this case is present in particular in the case of the transport of edge-perforated continuous form paper in printing devices, where the continuous form paper is fed by a transport device of a print station driven by an electromotor. The transport device, driven by an electromotor, comprises here a platen, driven by an electromotor, and pin-feed tractor wheels, which engage into the edge perforation of the continuous form paper. Based on the tolerances of the feed mechanism of an electromotor of the transport device, of the platen as well as of a slippage occurring between the platen and the continuous form paper in connection with a friction drive, the position to be headed for and reached of the continuous form paper shifts with each advance, relative to the upper edge of the continuous form paper, more and more away from the desired position. The therewith associated continuous increase of the positioning error has only an insignificant influence in case of a single sheet because of the smaller paper length in contrast to the continuous form paper. In case of printing on continuous form paper, the positioning error has therefore to be compensated. The positioning error becomes increasingly noticeable and visible, in particular where the continuous form paper is a printed blank form paper.
A mechanical paper tractor is known from the German printed patent document DE-A1-3,819,848, where for example a pin-feed tractor wheel, disposed on one side and driven by a motor, engages into the edge perforation of the continuous form paper for the transport of continuous form paper. If the pin-feed tractor wheel, coupled with the motor, for example a step motor, is driven with a drive shaft, then the continuous form paper is advanced and thereby moved over a platen past a print head. Very small tolerances are required in the mechanics of the known mechanical tractor in order to maintain the positioning error as small as possible. In addition, the mechanical paper tractor is associated with the disadvantage that a deviation, occurring based on the tolerances between the reached print position and the desired position, cannot be compensated.
Signals are given to an automatic control device, based on which the advance of the continuous form paper can be automatically controlled. In an exemplified embodiment of the device or, respectively, of the method, the distance between two neighboring edge perforation holes is determined for generating the signal given to an automatic control device, wherein the number of motor steps, determined for the path distance covered between two edge perforation holes during the relative motion of the continuous form paper relative to the optoelectronic scanning device, is compared to a theoretical number of motor steps. Dependent on this comparison result, a positioning error of the continuous form paper, resulting from the discrepancy between the actual and theoretical number of motor steps within a perforation hole distance during the relative motion of the continuous form paper relative to the optoelectronic scanning device, is corrected immediately upon surpassing a preset value. In addition, a number of motor steps is determined at the start of the edge perforation hole scanning of the continuous form paper, wherein said number of motor steps corresponds to the deviation of the optoelectronic scanning device based on a start position of the continuous form paper during the edge perforation hole scanning. The determined number of motor steps is again taken into consideration after scanning the edge perforation holes belonging to one sheet of the continuous form paper.
A transport device including a thrust tractor pair is disclosed in U.S. Pat. No. 5,061,096, where the thrust tractor pair is disposed in transport direction in front of a substrate support and is furnished with at least one driven gear wheel.
A drive mechanism for advancing paper in printing apparatus is shown in U.S. Pat. No. 4,577,849 to Watanabe.
A system for transporting web-shaped recording substrates in printing devices is disclosed in copending United States continuation-in-part-application to PCT application PCT/DE91/00197 filed Mar. 1, 1991.